Pigments are used to impart color and opacity to products such as paints, plastics, inks, glazes, and paper. Ideally, a pigment should have: strength, that is, only small amounts should be required to tint a white pigment; intensity, that is, purity of shade or absence of dullness or greyness; light-fastness, that is, resistance to color change when exposed to sunlight; resistance to bleed, that is, low migration of color in the pigmented article; and thermal stability. In addition, for many applications, it is desirable that a pigment have good hiding power, that is, ability to efficiently opacify the pigmented article.
Both inorganic and organic pigments are in common use. Inorganic pigments generally have the advantage of high opacity, high thermal stability, an ability to form bright, deep shade colors, and excellent bleed resistance. Typical inorganic pigments having yellow, orange, or red colors are lead chromate (PbCrO.sub.4), nickel titanate (NiTiO.sub.3), cadmium sulfide (CdS), and iron oxides (FeOOH and Fe.sub.2 O.sub.3).
Concerns presently are being manifested in the art and in the trade regarding the toxicity and, particularly, the carcinogenicity of commonly used pigments and heavy metals. The need exists, therefore, for nontoxic pigments with high hiding power and intensity and good strength, lightfastness, bleed resistance, and thermal stability.
The compounds BiOCl, BiOBr, and BiOI are known in the literature. They crystallize in the tetragonal PbFCl structure with space group P4.sub.n /mm (Bannister, Min. Mag. 24, 49 (1935)). BiOCl and BiOBr are colorless. BiOCl has long been used as a nacreous pigment in cosmetics. The nacreous or pearlescent effect is intensified by precipitating the BiOCl on mica or clays (U.S. Pat. Nos. 2,974,053, 3,788,871 and 3,917,671). The nontoxic qualities of BiOCl are attested to by its use in the coloring of externally-applied drugs and in cosmetics (Federal Register 42, #190, 52394, 1977; CA87:1893354). BiOI has been disclosed as a coral red nacreous pigment (M. Hunsdiecker, Fr. Patent 794,235).